Between 1969 and 1972, Apollo astronauts brought just under 842 pounds of rocks and regolith back from the Moon. In 1985, engineers at the University of Wisconsin discovered significant amounts of Helium-3 in the lunar soil.

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Helium-3 is a stable isotope of helium -- the gas we use to fill party balloons with -- and is notable because it's missing a neutron, an important property that means we can used it in nuclear fusion reactions to produce clean energy. Unfortunately, our most plentiful stores of the isotope are a quarter of a million miles away.

Current nuclear power plants use fission reactors, splitting uranium nuclei to release energy. This heat turns water into steam that drives a turbine to produce electricity. Unfortunately, radioactivity, spent nuclear fuel reprocessed into uranium, plutonium, and radioactive waste are by-products of this reaction.

To get away from fission power, scientists have been working on nuclear fusion energy.

Nuclear fusion is the same reaction that fuels the sun; high temperatures and dense concentrations of gas allow positively-charged nuclei to get close enough to each other that the attractive nuclear force overcomes the repulsive electrical force. They fuse, producing new elements and energy. Helium as the fuel in this type of reaction can provide energy without radioactivity and nuclear by-products.

Fusion reactors fueled by tritium and deuterium -- both isotopes of helium -- lose more energy than they produce, making them poor fuel sources. But fusion reactions between Helium-3 and deuterium, which creates normal helium and a proton without a neutron, wastes less energy. It's the proton that's important; manipulating it in an electric field produces energy. The Helium-3 fusion process is about 70 percent efficient compared to coal and natural gas, which are only about 20 percent efficient.

So we know how to harness the energy potential in Helium-3 (even though the technology to do so efficiently is a few years off), we just don't have enough of it on Earth to make it a viable energy source. That's because Helium-3 is carried by the solar wind and has a hard time getting through our planet's magnetic field.

We can make it -- tritium, hydrogen with two neutrons, and deuterium, hydrogen with an extra neutron, both decay into Helium-3. It's also a byproduct of nuclear weapons testing. But this still isn't enough. The United States' entire Helium-3 reserve is a little under 65 pounds; a country this size would need about 50,000 pounds -- 25 tons -- for a year's worth of power.

The moon's lack of magnetic field means Helium-3 can build up on its surface. As Apollo 17's lunar module pilot Harrison "Jack" Schmitt sees it, there are very few disadvantages to mining Helium-3 from the moon. Aside from it being a hard thing to do. Hard, but not impossible.